Development of the in-situ cosmogenic 14C method for studying geomorphic processes

本研究将完成在地球表面常见物质中原位测量宇宙成因14C（半衰期=5730年）的实验方法的开发，并为研究一些具有挑战性的地貌问题的方法进行初步研究。在美国国家科学基金会的资助下，我们已经掌握了一种实验方法，有可能提供准确的、无污染的（来自环境的）14C浓度测量。代替常用的高温熔合法，我们开发了一种真空湿法来测定矿物石英中CO和CO2相中的原位14C浓度。环境中的14C不影响14CO的测量，但在14CO2的测量中不规律地出现污染。由于原位14C的60%以上出现在CO相中，因此通过测量其在CO相中活性来测定原位14C是方便的。在第一年，工作将主要针对开发一种强大的实验技术，以准确测量x104原子14C/g石英的原位14C浓度，并确定不同海拔和纬度石英的原位14C产量。后者是其应用的先决条件。在第二年，我们建议对一些选定的地貌问题进行原位14C方法的一些测试应用。在短短一年的时间里，似乎很难解决如此具有挑战性的问题，但我们之所以能够做到这一点，是因为我们有一系列不同的地貌样本，在这些样本中，我们小组早些时候已经进行了宇宙成因10Be和26Al的研究。我们认为，在各种原位放射性核素中，14C应该为表征地貌表面提供最方便的方法，为使用其他原位放射性核素（例如36Cl， 26Al和10Be）以及稳定核素21Ne进行进一步分析提供坚实的基础。我们非常渴望完成实验方法的开发，以可靠地原位测量地表样品中的宇宙成因14C，并证明其在地貌学中的实用性。//

This research will complete the development of an experimental method for measuring in situ cosmogenic 14C (half-life=5730 yrs) in common surficial terrestrial materials, and for making pilot studies of the method for investigating a few challenging geomorphic problems. From the work carried out to date under NSF grant, we have in hand an experimental method, potentially capable of providing accurate and contamination-free (from environmental 14C) measurements of 14C concentrations. Instead of the commonly adopted method of high temperature fusion, we developed a wet in vacuo method to determine the in situ 14C concentrations in the CO and CO2 phases in the mineral quartz. Environmental 14C does not affect the 14CO measurements, but appears erratically as contamination in the 14CO2 measurement. Since more than 60% of in situ 14C appears in the CO phase, it is convenient to assay in situ 14C by measuring its activity in the CO phase. During the first year, work will primarily be directed towards the development of a robust experimental technique to accurately measure in situ 14C concentration at levels of x104 atoms 14C/g quartz, and the determination of in situ 14C production rates in quartz at different altitudes and latitudes. The latter is a prerequisite for its applications. In the second year, we propose to make some test applications of the in situ 14C method to a number of selected geomorphic problems. It may appear that little can be done in such challenging problems in just one year but we are able to do this because we have at a suite of diverse geomorphic samples in which in situ cosmogenic 10Be and 26Al studies have made earlier in our group. We believe that among the various in situ radionuclides, 14C should offer the most convenient approach for characterizing geomorphic surfaces, providing a firm basis for decisions for further analyses using other in situ radionuclides, e.g. 36Cl, 26Al and 10Be, and the stable nuclide, 21Ne. We are very anxious to complete the development of the experimental method to reliably measure in situ cosmogenic 14C in surficial samples, and to demonstrate its usefulness in geomorphology. //